The present invention relates to exposure systems for optically transferring patterns onto an object such as a wafer through a projection optical system, and more particularly to an alignment apparatus suitable for such an exposure system.
For manufacturing a semiconductor device, the overlapping exposure processes of a number of fine patterns having sizes below 0.5 .mu.m are required with respect to a wafer. An important problem in such exposure processes relates to the need for alignment between the number of fine patterns which requires the accuracy below 0.1 .mu.m. One known approach to resolution is disposed in the Japanese Patent Provisional Publication No. 63-78004 where a pair of first gratings on a reticle divide alight beam from a light source into a plurality of beams to selectively obtain a predetermined spectrum to project it onto a pair of second gratings on a wafer, and the second gratings are further exposed to two light beams so as to obtain two diffraction lights due to interference so that the two diffraction lights are compared in intensity with each other to effect the alignment between the reticle and the wafer.
However, there is a problem which arises with such an alignment arrangement in that the arrangement is effective only in the case that a projection optical system has high imaging performances with respect to both the exposure light and alignment light. For example, the glass material for constituting a refraction optical system is restricted in the case of using the ultraviolet light due to an excimer laser or the like as the exposure light and hence difficulty is encountered to constitute an achromatic projection optical system in which the chromatic aberration is corrected. Thus, the chromatic-aberration-corrected achromatic projection optical system is designed to sufficiently effect the color correction only in terms of the exposure light wavelength, thereby making it difficult to constitute the alignment system where the alignment light passes through a projection lens because an extremely great chromatic aberration occurs in terms of light with different wavelengths. If arranging the alightment system such that the alignment light does not pass through the projection lens, the projection lens is required to be arranged to cover the wafer whereby the exposure position of the wafer is greatly separated from the alignment position to lower the overlapping accuracy between a number of fine patterns.